Cryogenic distillation can involve moderate subambient temperatures within the range of from about 20.degree. F. to -100.degree. F. for applications such as natural gas processing, and much lower subambient temperatures within the range of from about -280.degree. F. to -320.degree. F. for applications such as air separation. For each application, the feed is passed into a distillation column and separated by providing refrigeration for the top of the column and heat addition at the bottom of the column. Often the necessary refrigeration and heat addition for the column separation are provided by a recirculating heat exchange fluid or heat pump which extracts the heat at a low subambient temperature and adds the equivalent heat at a higher subambient temperature. Since heat leak into a cryogenic distillation plant will occur, and since refrigeration may be removed from the plant in the form of cryogenic liquid product, it is generally necessary to add net refrigeration to a cryogenic distillation plant in order to maintain operation. Refrigeration may be added, for example, by the addition of low temperature liquid or by the turboexpansion of a pressurized process stream.
The addition of refrigeration to a cryogenic distillation plant is costly. It is desirable to have a cryogenic distillation method which can operate without the need for added refrigeration from outside the system.
Accordingly it is an object of this invention to provide a cryogenic distillation method wherein refrigeration requirements are compensated for by internal generation with reduced need for added refrigeration from outside the system.